The present invention relates to solenoid operated valves of the spool type which operate to provide a regulated pressure signal to a control signal outlet port by bleeding fluid to an exhaust or sump port from the valving chamber. It has been found that such valves have a tendency to reach instability at certain pressure signal levels and spool positions with respect to the ports; and, it has been found desirable to provide a reservoir or chamber of fluid at the end of the spool to dampen spool movement and minimize the instability.
Such valves have found application in the control of the shifting functions of an automatically shifted speed change power transmission such as utilized in passenger car and light truck motor vehicles. In such transmission applications, the valve body is mounted such that the spool motion is generally along a horizontally oriented axis; and, the spool dampening reservoir supplied with fluid by immersion in the transmission sump. However, during vehicle motion such as rapid acceleration, climbing or descending a hill, deceleration or cornering, the fluid in the sump may be moved away from the dampening reservoir port so as to expose the reservoir port and permit fluid to drain from the dampening reservoir. This occurrence can result in air entering the reservoir such that, upon return of the fluid to the inlet, the reservoir is not filled and dampening of the spool movement is disrupted.
Accordingly, it has been desired to provide a way of preventing the fluid dampening reservoir of a solenoid operated spool valve from becoming starved and fluid draining therefrom such as occurs during movement of the fluid sump.